Planetary gear assemblies, also referred to as epicyclic gear assemblies, are widely used in transmission systems. With reference to FIGS. 1A and 1B, a planetary gear generally designated PG is shown comprising a sun gear S, a ring gear R, and a planet carrier C carrying four planet gears P. Each of the sun gear S, planet gears P and ring gear R is formed with gear teeth (not shown) on the outer circumference thereof.
The sun gear S, ring gear R and planet carrier C are arranged concentrically about a central axis X in a manner allowing rotation thereof with respect to one another. Each of the planet gears P has an auxiliary axis Y, and is so mounted on the carrier C as to allow rotation thereof about its auxiliary axis Y. In assembly, the teeth of each planet gear P mesh with the teeth of the sun gear S on one hand, and with the teeth of the ring gear Ron the other hand.
The planet carrier C, planets P, ring gear R, sun gear S and constitute a group of gear elements, co-axially arranged about a central axis X. In operation, one of the elements of the group, e.g. planet carrier P, is held stationary, another element, e.g. sun gear S is connected to an input drive to facilitate rotary motion thereof, and rotary motion is thereby imparted to the third element and/or the planet gears P as output. Thus, the planetary gear system may operate at least at any one of the operational modes described in the following table:
TABLE 1Operational modeStationaryInputOutput1Ring gearPlanet carrierSun gear2Planet carrierRing gearSun gear3Ring gearSun gearPlanet carrier4Sun gearRing gearPlanet carrier5Planet carrierSun gearRing gear6Sun gearPlanet carrierRing gear
The transmission ratio D between the input rotary speed and the output rotary speed is determined according to the number of gear teeth of each of the sun gear S, planet gears P, planet carrier C and ring gear R.
Planetary gear assemblies are commonly used in automatic transmissions, for example for automotive vehicles, since they conveniently permit gear ratios to be smoothly changed by merely holding one of the components stationary.
Planetary gear assemblies are used in a large number of other applications for driving various types of electrical devices, such as milling machines, drilling tools, etc.
In particular, planetary gear assemblies are used in automotive vehicles. In this case, the planetary gear assembly forms a part of a hydraulic transmission. In such transmissions, a fluid coupling, or torque converter, and planetary gear sets are used to provide a wide range of torque multiplication.
With reference to FIG. 1C, a standard fluid coupling usually comprises an impeller CI and a rotor CR each having a plurality of blades adapted to circulate a fluid when set in rotary motion about their central axis X. The arrangement is such that in the presence of a fluid medium, for example oil, rotation of the impeller CI entails rotation of the rotor CR and vice versa due to circulation of the oil between the two.
In the automotive field, as well as in other fields of motorized operations, the behavior of the operated system in its initial stage is very important, i.e. behavior of the system from the moment the motor is turned on and until it reaches its desired RPM. For example, in a vehicle, it is desired to quickly accelerate at the beginning of the ride to a desired speed.
In most fluid couplings comprising an impeller CI and rotor CR (sometimes referred to as a ‘turbine’), the fluid coupling encounters a problem when the impeller begins rotating. This is caused mainly because the rotor hasn't picked up enough speed yet, whereby the oil circulated by the impeller and passing through the blades of the rotor tends to exit the blades of the rotor and impact the blades of the impeller in a manner applying a counter pressure in a direction opposite to the revolution of the impeller, thereby slowing it down. It is noted that this problem disappears when the impeller CI and rotor CR revolve at similar speeds.
This problem has been tackled by a variety of solutions, the most common of which is a stator. The stator arrangement comprises a plurality of blades, and is designed such that initially, when the impeller arrangement begins revolving at low speed, the blades of the stator direct the oil to impact the impeller blades at the proper angle. Thereafter, the rotor begins picking up speed and the stator is allowed to revolve along with the impeller and rotor, thereby providing a clean continuous start-up of the system.
In modern day vehicles, the stator arrangement is controlled by a computerized controller which commands the stator to begin revolving with the impeller and rotor once it detects that the rotor has reached a predetermined speed limit.